build: Factorize module compilation in (guix build compile).

* guix/build/compile.scm: New file.
* Makefile.am (MODULES): Add it.
* build-aux/compile-all.scm: Use it.
(warnings, file->module, load-module-file)
(%default-optimizations, %lightweight-optimizations)
(optimization-options, compile-file*): Remove.
<top level>: Use 'compile-files'.
* guix/build/pull.scm (%default-optimizations)
(%lightweight-optimizations, optimization-options): Remove.
(build-guix): Rewrite as a call to 'compile-files'.
* guix/discovery.scm (file-name->module-name): Export.